Method for improving the ability to recognize materials in an X-ray inspection system, and X-ray inspection system

ABSTRACT

A method for improving the ability to recognize materials in an X-ray inspection system is provided that includes the steps of recording at least two absorption X-ray images of an object to be examined at different energies, mathematically modeling the object by a number of layers assuming a particular material for each layer, wherein an absorption value describes the absorptivity of a layer, the number of layers is less than or equal to the number of X-ray images and at least one layer is assumed to be a material to be recognized during the inspection, decomposing the absorption value of each layer into a path-dependent factor and an energy-dependent factor, calculating the path-dependent factors for all layers from the absorption X-ray images using the absorption equation, calculating at least one synthetic image from the sum of all layers of the product of the absorption values and the weighting factors, evaluating the synthetic image.

This nonprovisional application is a continuation of InternationalApplication No. PCT/EP2008/006344, which was filed on Aug. 1, 2008, andwhich claims priority to German Patent Application No. DE 10 2007 042144.5, which was filed in Germany on Sep. 5, 2007, and which are bothherein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for improving the ability torecognize materials in an X-ray inspection system, and to an X-rayinspection system.

2. Description of the Background Art

It is known that in order to inspect objects, in particular pieces ofluggage, in respect of suspicious contents, X-ray inspection equipmentis used in which the objects to be inspected are irradiated by X-raybeams. For this, the X-ray inspection equipment has an X-ray source anddetectors in which the intensities of the attenuated radiation aredetected. Brightness values for a two-dimensional X-ray image displayedon a monitor are calculated from these intensities. Dangerous materialscan be recognized in this X-ray image.

In contrast to computed tomography scanners in the medical sector, X-rayinspection equipment for security checks or nondestructive testing hasfixed X-ray sources and detectors. Such X-ray inspection systemstherefore only irradiate each spatial point of the inspection object inone direction. Hence, a three-dimensional reconstruction of theinspection object, required for determining the density, cannot beachieved. Therefore, materials situated behind one another in the beampath cannot be easily recognized.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method thatimproves the ability to recognize materials in an X-ray inspectionsystem.

In an embodiment of the invention, at least two absorption X-ray imagesof an object to be inspected are firstly recorded at different energies.Here, the energies are advantageously selected such that the intensityof the X-ray radiation transmitted through the object is reduced bydifferent physical effects. It is preferable for two absorption X-rayimages to be recorded at two different energies according to the knowndual-energy method. Depending on the context, the term “X-ray image”refers to the measured X-ray intensities or the image calculatedtherefrom to be displayed on a monitor.

In general, the measured X-ray intensity is described by the absorptionequation:

$\begin{matrix}{{I(E)} = {\int_{E}{{{I_{0}(E)} \cdot {\mathbb{e}}^{- {\int\limits_{S}{{\mu{({E,{r{(s)}}})}}{\mathbb{d}s}}}}}\ {\mathbb{d}E}}}} & (1)\end{matrix}$

Here, I₀ refers to the energy-dependent intensity of the X-rayradiation, which would be incident on a detector if there were nomaterial between the X-ray source and the detector, and μ describes theabsorption coefficient depending on the spatial coordinate r(s) withinthe object. S describes the path of the X-ray radiation through theobject. Since X-ray inspection systems do not generally usemono-energetic X-ray radiation, there is integration in equation (1)over the energy spectrum of the X-ray radiation.

In a further method step, this is followed by the mathematical modelingof the object by a number of layers, assuming a particular material foreach layer. Here, the number of layers is less than or equal to thenumber of X-ray images and at least one layer is assumed to be amaterial to be recognized during the inspection. Since the materials areassumed to be homogeneous, μ is constant within a layer and theintegration over the path S is dispensed with. This results in theequation:

${{I(E)} = {\int_{E}{{{I_{0}(E)} \cdot {\mathbb{e}}^{- {\sum\limits_{M}\;{{\mu_{m}{(E)}} \cdot d_{m}}}}}\ {\mathbb{d}E}}}},$

where d_(m) is the thickness of the respective material, that is to saythe extent of the layer in the direction of the X-ray radiation. Mrefers to the number of layers in the model. The absorptivity of theobject is divided between the layers during the modeling; theabsorptivity of a layer is described by an absorption value μ_(m)·d_(m).The sum of the absorption values of the layers corresponds to theabsorption value of the entire object:

${\int\limits_{S}{{\mu( {E,{r(s)}} )}{\mathbb{d}s}}} = {\sum\limits_{M}\;{{\mu_{m}(E)} \cdot {d_{m}.}}}$

Furthermore, the absorption value of each layer is decomposed into apath-dependent factor and an energy-dependent factor. The path-dependentfactor does not depend on the energy, and the energy-dependent factordoes not depend on the path. Hence, the modeling proceeds on the basisof the reformulated absorption equation:

$\begin{matrix}{{I(E)} = {\int_{E}{{{I_{0}(E)} \cdot {\mathbb{e}}^{- {\sum\limits_{M}\;{{\tau_{m}{(E)}}{\delta_{m}{(S)}}}}}}\ {{\mathbb{d}E}.}}}} & (2)\end{matrix}$

The energy-dependent factor τ_(m) can correspond to the mass attenuationcoefficient and the path-dependent factor δ_(m) preferably correspondsto the density along the path through the material. In general, δ_(m)depends on the path S and is described by

$\begin{matrix}{{\delta_{m}(S)} = {\int_{S}{{\rho_{m}( {r(s)} )}\ {{\mathbb{d}s}.}}}} & (3)\end{matrix}$

Since the layers are assumed to be made of homogeneous materials,equation (3) simplifies toδ_(m)(S)=ρ_(m) ·d _(m)  (4)

A next step calculates the unknown path-dependent factors δ_(m) of alllayers from the absorption X-ray images using the absorption equation(2). The number of unknowns corresponds to the number of assumed layerswithin the scope of modeling the object. Each absorption X-ray imageprovides an equation for determining these unknowns; τ_(m) and ρ_(m) areknown for the assumed materials. Since the number of equations has to begreater than or equal to the number of unknowns, the number of layershas to be less than or equal to the number of X-ray images. Here, apriori knowledge of the object to be examined can replace one or moreX-ray images.

The thicknesses of the layers can be calculated from the path-dependentfactors using equation (4). The layers would have these thicknesses ifthe object to be examined consisted exclusively of the assumedmaterials. If the object contains other materials than those assumedduring the modeling, these other materials result in false contributionsto the path-dependent factors and thus to the thicknesses of a pluralityof layers. However, as will be shown below, this is unimportant forimproving the ability to recognize materials.

In a next method step, at least one synthetic image is calculated fromthe sum of all layers of the product of the absorption values and theweighting factors. By way of example, the synthesizing equation usedduring this calculation is

$\begin{matrix}{{I_{syn}(E)} = {\int_{E}{{{I_{0}(E)} \cdot {\mathbb{e}}^{- {\sum\limits_{M}\;{{w_{m} \cdot {\tau_{m}{(E)}}}{\delta_{m}{(S)}}}}}}\ {\mathbb{d}E}}}} & (5)\end{matrix}$

Thus, a new image is synthesized from the model of the object, theproperties of the assumed materials and the path-dependent factors ofthe layers. The selection of the weighting factors w_(m) affords thepossibility of designing the synthetic image such that an objectincluding a material to be recognized does not have a contour, or onlyhas a weak contour, in the synthetic image. If all weighting factors areselected to be 1, the synthetic image corresponds to a recordedabsorption X-ray image.

The final step includes evaluating the synthetic image. By way ofexample, the evaluation can be automated, with, for example, an alarmsounding if the object to be examined comprises a material to berecognized. Alternatively or additionally, the evaluation is carried outby displaying the synthetic image on a monitor. Optionally, two or moresynthetic images are calculated using different weighting factors andare displayed one after another or next to one another on the monitor.For this, the X-ray inspection system operator switches between thesynthetic images using, for example, a button or a switch. In onerefinement of the invention, the weighting factors for at least onesynthetic image or all synthetic images can be set by the operator.

The absorption value of a layer with a material to be recognized can beweighted using the factor zero during the calculation of the syntheticimage. Accordingly, the synthetic image does not contain an absorptioncomponent that is associated with the material to be recognized duringthe modeling of the object. Accordingly, a contour of an object made ofthe material to be recognized, which contour becomes apparent in therecorded absorption X-ray images, is not contained in the syntheticimage. By way of example, the operating staff of the X-ray inspectionsystem deduces the presence of the material to be recognized in theobject by the lack of this contour.

In an embodiment of the invention, the weighting factors in thecalculation of the synthetic image depend on the position of the pixelin the X-ray image. Accordingly, w_(m)=w_(m)(x,y) holds true, with x andy being the coordinates of a pixel in the image. By way of example, thisaffords the possibility of suppressing the contours of items made ofdifferent materials in various regions of the synthetic image usingdifferent weighting factors and thus affords the possibility ofdetecting a plurality of materials to be recognized in a syntheticimage.

The absorption caused by a layer during the calculation of the syntheticimage is preferably colored as a function of the assumed material of thelayer. The coloring is preferably brought about based on the atomicnumber Z of the material. Thus, for example, metallic materials arecolored blue and organic materials are colored orange. This coloringaffords simpler detection of the sought after materials by the operatingstaff of the X-ray inspection system.

An X-ray inspection system according to the invention has means forcarrying out the method described above.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus, are not limitiveof the present invention, and wherein:

FIG. 1 shows a section through an object to be examined made of twomaterials;

FIG. 2 a shows the theoretical absorption thickness of the first assumedmaterial in the object from FIG. 1;

FIG. 2 b shows the theoretical absorption thickness of the secondassumed material in the object from FIG. 1;

FIG. 3 shows a section through an object with three materials;

FIG. 4 a shows the theoretical absorption thickness of the first assumedmaterial in the object from FIG. 3; and

FIG. 4 b shows the theoretical absorption thickness of the secondassumed material in the object from FIG. 3.

DETAILED DESCRIPTION

FIG. 1 illustrates a schematic sectional illustration of an object to beexamined that is composed of two materials. Material 1 is the plasticexplosive Semtex; material 2 is iron. The thickness of material 1 is 1.5mm; the thickness of material 2 is 1 mm. Four positions A, B, C and D,where X-ray detectors are arranged, are highlighted in an exemplaryfashion. When an absorption X-ray image is recorded, there is nomaterial between the (not illustrated) X-ray source in an X-rayinspection system and the X-ray detectors at positions A and B, there isonly material 2 between the X-ray source and the detector at position Cand both material 1 and material 2 between the X-ray source and theX-ray detector at position D. The X-ray detectors detect the X-rayradiation emitted by the X-ray source and attenuated by the object.

In a dual-energy X-ray inspection system, two absorption X-ray images ofthe object are recorded at two different energy spectra of the X-rayradiation. In the present example, the energy E_(L) of the X-rayradiation in the low-energy recording is between 20 keV and 70 keV; theenergy E_(H) in the high-energy recording is between 70 keV and 140 keV.The absorption X-ray images are two-dimensional images made up ofindividual pixels, wherein the brightness of each pixel corresponds tothe absorptivity of the object to be examined at this image position.The two absorption X-ray images provide both intensities I(E_(L)) andI(E_(H)) for each pixel.

Subsequently, the object is modeled mathematically using two layers. Inone layer, Semtex is assumed to be material 3; iron is assumed to bematerial 4 in the other layer. The intensity of the X-ray radiation,which impinges on an X-ray detector after being transmitted through themodeled object, is then calculated from the absorption equation (2) foreach pixel. If these theoretical intensities are equated to the measuredintensities, this results in the two equations

$\begin{matrix}{{{I( E_{L} )} = {\int_{E_{L}}{{{I_{0}( E_{L} )} \cdot {\mathbb{e}}^{{{- {\tau_{3}{(E_{L})}}} \cdot {\delta_{3}{(S)}}} - {{\tau_{4}{(E_{L})}} \cdot {\delta_{4}{(S)}}}}}\ {\mathbb{d}E}}}}{and}} & (6) \\{{I( E_{H} )} = {\int_{E_{H}}{{{I_{0}( E_{H} )} \cdot {\mathbb{e}}^{{{- {\tau_{3}{(E_{H})}}} \cdot {\delta_{3}{(S)}}} - {{\tau_{4}{(E_{H})}} \cdot {\delta_{4}{(S)}}}}}\ {\mathbb{d}E}}}} & (7)\end{matrix}$

with the two unknowns δ₃ and δ₄. Here, I₀ refers to the known intensityof the X-ray radiation that would impinge on a detector if there were nomaterial between X-ray source and detector. τ₃ and τ₄ are knownmaterial-specific mass attenuation coefficients, which are independentof the length of the path S of the X-ray radiation through the materialbut do depend on the energy of the radiation. The densities along thepath δ₃ and δ₄ depend on the length of the path S of the X-ray radiationthrough the material, but are independent of the energy of theradiation.

The two unknowns δ₃ and δ₄ can be calculated using equations (6) and(7). They are connected to the thicknesses of the layers via equations(3) and (4). For illustrative purposes, these thicknesses are usedinstead of the densities along the path δ in the figures and thefollowing embodiments.

The calculated thickness of material 3 is illustrated in FIG. 2 a; thecalculated thickness of material 4 is illustrated in FIG. 2 b. If, as inthe present example, the assumed materials exactly correspond to thematerials from which the object is composed, the calculated thicknessesexactly correspond to the actual thicknesses of the materials in theobject. If a plurality of items in the object to be examined that aremade of the same material are situated in the propagation direction S ofthe X-ray radiation, these are combined to form a common layer duringthe modeling of the object. This also holds true if these items are notdirectly adjacent in the object.

In a next step, at least one synthetic image is calculated using thesynthesizing equation (5). Here, it has the following form:

I_(syn) = ∫_(E)I₀(E) ⋅ 𝕖^(−w₃τ₃(E) ⋅ δ₃(S) − w₄τ₄(E) ⋅ δ₄(S)) 𝕕E

The absorption values τ₃(E)·δ₃(S) and τ₄(E)·δ₄(S) are multiplied by theweighting factors w₃ and w₄ and summed. Multiplied by the factor −1, thesum forms the exponent of the exponential function in the absorptionequation. In the present exemplary embodiment, the weighting factors w₃and w₄ are selected to be constant for the entire image, that is to sayfor all pixels with arbitrary coordinates x and y. The calculatedintensities I_(syn) are converted in a known fashion into atwo-dimensional image to be displayed on a monitor. Optionally, thecomponents of the individual layers of the overall absorption arecolored as a function of the assumed material of the layer.

If the weighting factor w₄ is set to equal zero, the synthetic image isbased exclusively on the absorption component through the assumedmaterial 3 (Semtex in the present case). FIG. 2 a makes it clear thatthe assumed material 4, which exactly corresponds to the actual material2, does not provide any image component. A contour, which the material 2leaves in one of the recorded absorption X-ray images, is missingentirely in the synthetic image. This allows the deduction to be madethat the assumed material 4, i.e. iron, is contained within the object.If, by contrast, the weighting factor w₃ is set to zero, the syntheticimage is based exclusively on the absorption components of the assumedmaterial 4. A contour of the material 1, which contour can bedistinguished in an absorption X-ray image, is not contained in thesynthetic image, as a result of which it is possible to deduce thepresence of the assumed material 3 in the object.

FIG. 3 shows a section through an object made of 3 materials. Materials5 and 6 and the thicknesses thereof correspond to materials 1 and 2 inFIG. 1. Additionally, the object comprises a 2 mm thick material 7. Thematerial 7 is situated in the beam path between the X-ray source and thedetectors at positions B, C and D, but not between the X-ray source andthe detector at position A.

The object is once again described by a model made of two layers. Forthis, the equation

I(E) = ∫_(E)I₀(E) ⋅ 𝕖^(−τ₈(E) ⋅ δ₈(S) − τ₉(E) ⋅ δ₉(S)) 𝕕E

is used. For this model, material 8 is assumed to be Semtex and material9 is assumed to be iron, as a result of which these materials correspondto two of the materials actually present in the object. δ₈ and δ₉ arecalculated analogously to the first exemplary embodiment.

Since the proposed two-layer model cannot reproduce an object made ofthree materials correctly, the calculated thicknesses do not correspondto the actual thicknesses of the materials 5 and 6 in the object, evenif the materials 8 and 9 are assumed correctly. Rather, the material 7results in a component in both calculated thicknesses since it does notcorrespond exactly to any of the assumed materials. This results in thethickness profiles of the material 8 as shown in FIG. 4 a and thematerial 9 as shown in FIG. 4 b.

The absorption of the X-ray radiation through the material 7 measured bythe X-ray detector at position B corresponds to that absorption thatwould be caused by 1.2 mm of material 8, i.e. Semtex, and 0.2 mm ofmaterial 9, i.e. iron. These mentioned thicknesses form an incorrectoffset at positions B, C and D. For Semtex as assumed material 8, whichcorresponds to the actual material 5, there is an additional, correctthickness component of 1.5 mm at position D and hence there is anoverall calculated thickness of 2.7 mm. For the material 9, which wascorrectly assumed to be the actual material 6 iron, there is anadditional thickness component at positions C and D of 1 mm as a resultof the object and hence an overall calculated thickness of 1.2 mm.

It can be seen from the profiles of the calculated thicknesses in FIGS.4 a and 4 b that, although the third material 7 results in an additionalcomponent of the thicknesses of the assumed materials, the real contourof an item in the object and having the respective material is stillreproduced correctly.

When the weighting factor w₉ is selected to be zero, the synthetic imagedoes not contain contours from iron as material 6, although it doescontain contours as a result of materials 5 and 7. By contrast, if theweighting factor w₈ is selected to equal zero, the synthetic image doesnot contain any contours caused by Semtex. The lack of a contourcompared to a recorded absorption X-ray image can thus allow thepresence of the assumed material in the object to be deduced, even ifthe object contains more or different materials than those taken accountof by the mathematical model.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are to beincluded within the scope of the following claims.

1. A method for recognizing materials in an X-ray inspection system, themethod comprising: irradiating an object to be examined; recording atleast two absorption X-ray images, of the object to be examined, atdifferent energies; mathematically modeling the object by a number oflayers assuming a particular material for each layer, wherein anabsorption value describes an absorptivity of a layer, the number oflayers being less than or equal to the number of X-ray images, and atleast one layer being assumed to be a material to be recognized duringthe inspection; decomposing the absorption value of each layer into apath-dependent factor and an energy-dependent factor; calculating thepath-dependent factors for all layers from the absorption X-ray imagesusing the absorption equation; calculating at least one synthetic imagefrom a sum of all layers of the product of the absorption values andselected weighting factors; and evaluating the synthetic image.
 2. Themethod as claimed in claim 1, wherein two absorption X-ray images arerecorded at two energies.
 3. The method as claimed in claim 1, whereinthe absorption value of a layer with a material to be recognized isweighted by a factor zero during calculation of the synthetic image. 4.The method as claimed in claim 1, wherein the weighting factors in thecalculation of the synthetic image depend on a position of a pixel inthe X-ray image.
 5. The method as claimed in claim 1, wherein, duringcalculation of the synthetic image, the absorption caused by a layer iscolored as a function of the material assumed for the layer.
 6. Themethod as claimed in claim 1, wherein the energy-dependent factor is amass attenuation coefficient and the path-dependent factor is a densityalong the path through the material.
 7. The method as claimed in claim1, wherein the different energies are selected such that an intensity ofX-ray radiation transmitted through the object to be examined isreduced.
 8. The method as claimed in claim 1, wherein the material ofthe layers is assumed to be homogeneous.
 9. The method as claimed inclaim 1, further comprising calculating a thickness of the layers fromthe path-dependent factors.
 10. The method as claimed in claim 1,wherein said calculating at least one synthetic image comprisescalculating a plurality of synthetic images using different weightingfactors.
 11. The method as claimed in claim 1, wherein the differentenergies comprise a low-energy and a high-energy.
 12. The method asclaimed in claim 1, wherein the different energies comprise a firstenergy between 20 keV and 70 keV and a second energy between 70 keV and140 keV.
 13. The method as claimed in claim 1, wherein the absorptiveX-ray images are two-dimensional images made up of individual pixels,and wherein a brightness of each individual pixel corresponds to theabsorptivity of the object to be examined.
 14. An X-ray inspectionsystem comprising: an X-ray source; and a detector configured to detectan x-ray generated by the X-ray source, wherein the X-ray inspectionsystem is configured to: records at least two absorption X-ray images,of an object to be examined, at different energies; mathematically modelthe object by a number of layers assuming a particular material for eachlayer, wherein an absorption value describes an absorptivity of a layer,the number of layers being less than or equal to the number of X-rayimages, and at least one layer being assumed to be a material to berecognized during the inspection; decompose the absorption value of eachlayer into a path-dependent factor and an energy-dependent factor;calculate the path-dependent factors for all layers from the absorptionX-ray images using the absorption equation; calculate at least onesynthetic image from a sum of all layers of the product of theabsorption values and selected weighting factors; and evaluate thesynthetic image.